An object or substance by which something is mediated, such as
1. (Science: anatomy) A structure of the nervous system that transmits impulses eliciting a specific response.
2. (Science: chemistry) A chemical substance (transmitter substance) that induces activity in an excitable tissue, such as nerve or muscle, or
3. (Science: cell biology) A substance released from cells as the result of the interaction of antigen with antibody or by the action of antigen with a sensitised lymphocyte.